Most of us don’t give much thought to the humble comma. It’s just something we scatter about when the moment feels right, isn’t it? But as a result it has become what Bill Bryson (whose Troublesome Words is one of my literary bibles) calls ‘the most abused of punctuation marks and one of the worst offenders of any kind in the English language’.
One of the most common abuses goes something like this:
Tom wore blue jeans and a white shirt, his shoes were from Marks & Spencer’s.
This is what is sometimes called a run-on sentence. The two parts separated by the comma are related because it’s all to do with what he’s wearing – but in fact they are two self-contained statements and the pesky comma is the wrong tool for the job. You could use a full stop (period), although sometimes that might a sentence sound rather staccato so you could use a linking word to get round a problem:
Tom arrived on Monday and Dick came on Wednesday, Harry couldn’t make it
is wrong and needs a full stop instead of a comma after ‘Wednesday’. But it could also be turned into a single sentence by using linking words such as ‘and’ and ‘but’:
Tom arrived on Monday and Dick on Wednesday, but Harry couldn’t make it.
I suspect it’s one of those things you either get or you don’t, and that if you don’t it’s hard to make the distinction or see what all the fuss is about. One test occurs to me (though I’ve only just dreamt up so might not work in every instance) is that if both parts of a sentence linked by a comma work and make sense when split into two by the use of a full stop instead, then it’s probably a run-on sentence.